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#glycosylation

6 public questions tagged with this topic.

Fertilization in mammals-iv

Glycosylation of ZP proteins affects:

Zona pellucida comprises sulfated glycoproteins ZP1-ZP4 bearing extensive N- and O-linked glycans that regulate mechanical stiffness and ligand function. Glycan moieties present terminal sialic acid and specific oligosaccharide epitopes recognized by sperm plasma membrane receptors such as galactosyltransferase, influencing species-restricted adhesion and triggering acrosome reaction. Altered glycosylation modifies zona solubility, protease resistance and sperm binding avidity without affecting sperm motility or implantation directly. Deglycosylation experiments reduce but may not abolish binding, underscoring glycans as key modulators of primary sperm-zona recognition and polyspermy block after fertilization.

Ref: Avella et al., J Cell Biol 2014: Zona pellucida glycosylation modulating sperm recognition and species-specific binding.

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Animal Tissue Culture 3

Glycosylation of recombinant proteins is best achieved using:

Mammalian expression systems are best source for producing therapeutic glycoproteins that require authentic post-translational modifications to be biologically active and non-immunogenic. Enzymes localized in endoplasmic reticulum and Golgi apparatus catalyze N-linked glycosylation at Asn-X-Ser/Thr motifs via oligosaccharyltransferase, trimming by glucosidases, addition of complex sialylated structures by sialyltransferases, O-glycosylation, formation of disulfide bonds by protein disulfide isomerase and Ero1, gamma-carboxylation, and proper folding assisted by chaperones BiP, calnexin, calreticulin. Bacterial systems such as Escherichia coli lack glycosylation machinery, produce proteins as insoluble inclusion bodies needing refolding, and add non-human metabolites. Yeast Saccharomyces can glycosylate but hypermannosylates leading to rapid clearance and immunogenicity. Cell-free extracts have limited capacity for disulfide bond formation and complex modifications. Therefore Chinese hamster ovary CHO, HEK293, and BHK lines dominate biopharmaceutical manufacturing of monoclonal antibodies, erythropoietin, clotting factors VIII/IX, and viral vectors where glycan profile dictates half-life, efficacy, and safety, justifying rigorous glycoanalysis during process development. Bioengineering efforts aim to humanize glycosylation pathways in CHO to produce afucosylated antibodies enhancing effector function. This knowledge strengthens laboratory safety, protocol reproducibility, and regulatory compliance critical for translational research and clinical applications, ensuring reliable data and workforce protection.

Ref: Wurm FM Nat Biotechnol 2004 CHO mammalian cell protein production; Lodish MBoC 8th Ed Ch.13 Protein glycosylation and ER quality control.

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Cell Organelles

The Golgi apparatus is involved in which type of glycosylation?

Protein glycosylation diversity arises from distinct enzyme locations. N-linked glycosylation defined by oligosaccharide attachment to asparagine within Asn-X-Ser/Thr consensus begins cotranslationally in ER where oligosaccharyltransferase transfers preassembled Glc3Man9GlcNAc2 from dolichol, then trimmed by glucosidases. Extension into complex types continues in Golgi but core attachment ER-specific. O-linked glycosylation where N-acetylgalactosamine alpha linked to serine threonine initiated by family of twenty polypeptide GalNAc transferases transferring GalNAc from UDP-GalNAc directly to protein, without lipid intermediate, occurs predominantly in cis and medial Golgi. Subsequent core synthesis by core one synthase adding galactose, core two GlcNAc transferase, sialyltransferases generating sialyl Tn antigens happens in trans Golgi. Mucins, proteoglycans and Notch receptors rely on Golgi O-glycosylation regulating adhesion and signaling. Phosphorylation by kinases cytosolic nuclear, acetylation by acetyltransferases nuclear cytosolic, therefore not Golgi glycosylation. Benzyl-GalNAc inhibits O-elongation demonstrating Golgi role in O-linked pathway essential for barrier and immune recognition. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Spiro Glycobiology; O-GalNAc transferase initiates O-glycosylation in Golgi, N-linked starts ER.

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Protein Sorting and Trafficking

What is the correct sequence of glycosylation steps in the Golgi?

Conversion of high-mannose precursors to complex N-glycans requires spatial separation of Golgi enzymes across cis, medial and trans cisternae enforcing substrate channeling and ordered maturation preventing futile cycles and ectopic branching errors. Early cis-Golgi contains alpha-mannosidase I trimming Man9GlcNAc2 to Man5GlcNAc2 removing alpha1,2 mannoses requiring calcium cofactor and interaction with cargo receptors. Next medial-Golgi resident GlcNAc transferase I adds GlcNAc to alpha1,3 mannose branch generating GlcNAcMan5GlcNAc2 now high-affinity substrate for medial mannosidase II. Mannosidase II removes terminal alpha1,3 and alpha1,6 mannoses producing GlcNAcMan3GlcNAc2 core. GlcNAc transferase II adds second GlcNAc to alpha1,6 arm creating conserved biantennary GlcNAc2Man3GlcNAc2 platform for further elaboration. Subsequent steps include core fucosylation by FUT8, galactosylation by B4GalT1 and capping with sialic acid by ST3Gal and ST6Gal sialyltransferases in trans-Golgi finalizing complex glycans. Strict order ensures fidelity; reversing mannosidase II before GlcNAc transferase I blocks progression because enzyme specificity strictly depends on prior GlcNAc addition essential for hybrid formation and branched structures.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 15: Ordered Golgi N-glycan processing sequence.

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Protein Sorting and Trafficking

Which glycosylation type involves attachment to tryptophan residues?

Among glycosylation classes, attachment via carbon-carbon bond to tryptophan indole ring is chemically distinct and relatively rare. C-mannosylation involves α-D-mannopyranose linked to carbon-2 of indole of tryptophan within consensus W-X-X-W or W-X-X-C motif, catalyzed co-translationally inside ER lumen by multi-pass membrane enzymes DPY19L1 through L4 using dolichol-phosphate-mannose as high-energy donor substrate assembled on ER membrane. Modification increases hydrophobicity locally, stabilizes beta-propeller and thrombospondin type 1 repeat folds found in properdin, F-spondin, mindin, ADAMTS proteases and mucins, influences secretion efficiency and solubility. Unlike N-linked glycan attached to asparagine amide or O-linked via serine hydroxyl, C-linkage resists PNGase and O-glycosidases, providing structural reinforcement rather than bulky hydrophilic decoration. Defects in DPY19L1 cause developmental delay and eye anomalies. Golgi-mediated vesicular transport, actin filament association, mannose trimming for ERAD or direct lysosomal delivery refer to trafficking processes unrelated to tryptophan modification, underscoring C-mannosylation as unique post-translational enzymatic linkage expanding proteome chemical diversity beyond canonical N and O linkages and regulatory importance for signaling. Additional coordination with cellular stress pathways ensures fidelity, prevents aggregation, and links trafficking to growth control and proteostasis maintenance across diverse cell types and developmental stages.

Ref: Buettner et al., Mol Cell 72: 2018, C-Mannosylation of Tryptophan by DPY19.

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